The present invention relates to methods and devices for laser welding, particularly for the welding of structures made of sheet metal constituting assemblies or subassemblies of bodies or frames of motor-vehicles.
The present applicant has proposed for some time now (see, for example, the European patents Nos. EP 0440001 B1 and EP 0440002 B1 regarding the so-called “LASERGATE” system) devices for the laser welding of motor-vehicle structures. The use of laser welding for said applications did, however, not find a particularly wide diffusion immediately after its first proposal, in the early nineties. This is due principally to the fact that the experiments conducted with the first embodiments of laser-welding devices demonstrated the existence of a wide range of problems connected to said technology.
A first important problem derives from the widespread use, in the automotive field, of sheet steel provided with an outer zinc-coated protective layer. Said layer gives rise to the generation of zinc vapours during the laser-welding operation, which frequently render it problematical to obtain a good-quality weld.
The above problem has been addressed and illustrated extensively in the European patent applications Nos. EP 1238748 A1 and EP 1236535 A1, filed in the name of the present applicant, where there are illustrated devices that are able to overcome, in a simple and efficient way, the aforesaid technical obstacle, by guaranteeing ways of escape for the zinc vapours that are formed during the process of welding. Another solution to the problem constituted by zinc vapours has also been proposed in the Italian patent application No. TO2002A000760, filed in the name of the present applicant.
It, however, required some time for the aforesaid technical problem to be recognized, studied, and solved in a complete way, a fact that explains, at least in part, the length of the period of gestation of the use of laser welding in the automotive field.
Another important problem that it is necessary to take into account in the application of laser welding to the assembly of motor-vehicle structures is linked to the need to guarantee a high quality of assembly with reduced production times. A station for assembly of a motor-vehicle body or for its subassembly typically comprises a plurality of locating elements and clamping equipment that ensure correct positioning of the elements of sheet metal constituting the structure during the welding step. There exists, of course, a minimum limit to the number of pieces of clamping equipment that can be provided for said purpose, below which the geometry of the structure is not adequately guaranteed, with the consequence of an insufficient quality of the assembly operation. It follows that the welding station is relatively “crowded” by a set of clamping equipment, with the corresponding control devices for manoeuvring said equipment between an open, inoperative, condition and a closed, operative, condition. Added to this is the fact that, in the case of flexible welding stations, capable of operating on different types or models or versions of structure to be welded, the welding station is also provided with means for guiding and controlling different structures for supporting the clamping equipment, which are rapidly interchangeable with one another according to the type of body or subassembly that each time arrives in the welding station. The consequent relative complexity of the structure of the welding station and of its parts obviously renders more difficult the work of the manipulator robots that are used for carrying the welding means (electrical welding guns or yokes in the conventional case, laser heads in the case of laser welding) in the proximity of the various areas of the structure to be welded.
Both in the case of the traditional technology using electrical welding guns and in the case of laser welding, the robot must move successively into a series of areas of the structure to be welded for executing the welds that are assigned to it. Consequently, after the structure to be welded has arrived in the welding station, it must remain in said station for a time at least sufficient to enable each robot to perform all the welds assigned to it. Obviously, the time of stay in the welding station could be reduced by increasing the number of robots, but also in this case there exists a limit to said possibility, which is due both to reasons of costs, and to the fact that, above a certain number of robots, each of the robots becomes an obstacle to the operativeness of one or more robots adjacent to it.
On the other hand, the time used by each robot for making all the welds assigned is represented not only by the sum of the times necessary for making the various welds, but also by the time occupied on each occasion for coming into the area to be welded, and said time cannot be negligible, above all when the robot is forced to follow, for the purpose, a relatively tortuous path, it being necessary to prevent any interference whether with the parts of the structure to be welded or with the various pieces of clamping equipment engaged thereon.
It is necessary to consider, on the other hand, that, at the outset of the application of laser technology to the welding of motor-vehicle structures, the laser generators available were relatively less efficient and less powerful than the ones that are, instead, currently available. With the first-generation laser generators, it was in any case necessary to guarantee a position of the laser head carried by the robot that was relatively close to the structure to be welded, so that the application of laser technology did not yield particular advantages, from this standpoint, as compared to the traditional electric spot-welding techniques. With the currently available laser systems, instead, there are opened fresh, encouraging prospects in the direction of an important reduction in production times.
The idea that underlies said evolution and has formed the subject of initial experiments conducted by the present applicant consists in maintaining the laser head at a certain distance from the structure to be welded and in providing means that will enable focusing of the laser beam in different areas of the structure to be welded, without modifying the position of the laser head. This is, of course, exploited not only for moving the laser beam with respect to the structure to be welded in a given area, for the purpose of performing a welding stretch, or welding bead, but also and above all for welding different areas of the structure, without moving the head carried by the robot. A solution in this direction has been proposed by the present applicant in the European patent application No. EP1228835A1 (A system and method of remote laser welding), of which the present applicant is co-owner. Said known system is applied, however, to a “Cartesian” robot, not to a robot of an “anthropomorphic” type, and is “added”, and not integrated, in the robot.